In many of the present day semiconductor devices, aluminum is commonly used for interconnect lines and contacts. Aluminum offers a number of advantages in ease of fabrication. Nevertheless, when the aluminum wire size is decreased to smaller than 0.5 μm, concerns over electromigration resistance and the stress migration resistance of aluminum increase. In addition, when the feature size of an aluminum-based contact requires an aspect ratio of greater than 1:1, it is difficult to obtain planarization of the substrate during the application of the next insulating layer over the contact area of the substrate. As demands on transistor gate velocity and interconnect line transmission time increase, copper is becoming the material of choice for the next generation of interconnect lines and contacts. After all, the resistivity of copper is about 1.4 μΩ/cm, which is only about half of the resistivity of aluminum. Generally speaking, copper exhibits low corrosion, low electromigration and high conductivity. However, copper is not easily integrated into integrated circuit fabrication process flows.
For example, CMP techniques used to remove excess copper from the dielectric surface after deposition create problems. Copper is a soft material which can smear across the underlying surface during polishing. Dishing of the copper surface may occur during polishing due to deformation of the polishing pad into wide features. As a result of dishing, there is variation in the critical dimensions of conductive features. Moreover, particles from the slurry used during the CMP process may become bonded to the surface of the copper and other materials surrounding the location of the copper lines and contacts. The chemicals present in the slurry may corrode the copper, leading to increased resistivity and possibly even corrosion through an entire wire line thickness.
Although copper can be etched in a variety of ways, concerns are present. Copper may be plasma etched using a chlorine based etch chemistry at highly elevated temperatures. Ion milling or magnetron etching with argon or chlorine reagents may also be used. However, the use of halogens is disfavored for many reasons. Wet chemical etching of copper is also known, such as using HCl. When wet etch processes are employed, there is difficulty in controlling the etch profile of the features, as well as the etch rate. In particular, when the thickness of the copper layer being etched is comparable to the minimum pattern dimension, undercutting due to isotropic etching becomes a greater concern. In addition, there is extreme corrosion of the copper during the wet etch process itself. Wet etching often leaves the copper surface undesirably rough and pitted, complicating further processing.
Other concerns in utilizing copper in semiconductor processing applications involve difficulties in cleaning the copper. For instance, an electrical connection with other conductive components is often made to copper. Formation of such electrical connection frequently involves providing a conductive material over the surface of the copper. However, if the surface of the copper is partially or entirely covered with debris, the physical connection of the conductive material and the copper surface can be impaired. Such impairment can lead to attenuation of electrical current passing between the copper and the conductive material formed thereover.